1. Field of the Invention
This invention relates to the field of metallurgy, and particularly to the field of processing precipitation hardening aluminum alloys.
2. Description of the Prior Art
A fine grain size tends to improve the mechanical properties of many structural materials. Additionally, formability can be improved by elimination of "orange peel" structure, and superplasticity realized in many alloys by providing a fine grain structure. In some environments, some alloys have better corrosion resistance when their structure is fine grain rather than coarse grain. However, grain refinement is difficult to achieve in aluminum alloys, and most attempts to obtain a fine grain size by conventional mechanical working and recrystallization by heating have only resulted in the material recrystallizing to the original coarse grain size with large "pancake" shaped grains.
U.S. Pat. No. 4,092,181 to Paton and Hamilton describes a method of imparting a fine grain to precipitation hardening aluminum alloys. U.S. Pat. No. 4,222,797 to Hamilton, Mahoney, and Paton describes an improvement in the earlier method which utilizes an increased rate of forming to obtain finer grains. According to both these prior methods, an aluminum alloy is overaged to develop grain refining precipitates, mechanically worked to provide strain energy, and then heated to above its recrystallization temperature to recrystallize into a fine grain structure. The present invention utilizes these same process steps, but includes additional features which minimize the resultant grain size.